Ivascyn said Newport Beach, Calif-based Pimco believes the U.S. central bank's first rate hike in nine years will come before the end of the year, but not in September.

"The decision this week will be close," Ivascyn said. "Global weakness, uncertainty around consumer sentiment, inflation still running below target are reasons the Fed will not move this week."

Ivascyn's views come days after Pimco held its quarterly forum last week. Pimco managing directors as well as senior advisers including Bernanke and consultant Gene Sperling discussed trends for economies and markets over next 12 to 18 months.

Ivascyn's views are widely followed as he helps Pimco oversee assets that totaled more than $1.52 trillion as of June 30, mostly in fixed-income securities. Pimco is a unit of Munich-based Allianz SE .

The recent market turmoil, stemming from China's currency devaluation, has called into question the Fed's long-telegraphed plans to raise rates from near zero this year, possibly as soon as Thursday.

As of the end of August, the firm's $98.1 billion Pimco Total Return Fund had over 50 percent of its assets in mortgage-related assets and 19.19 percent in U.S.-government related securities. The fund also had 28.87 percent of its assets in emerging markets and 9.50 percent in investment-grade corporate credits and 5.40 percent in high-yield "junk" bonds.

The U.S.-government category may include nominal and inflation-protected Treasuries, Treasury futures and options, agencies, FDIC-guaranteed and government-guaranteed corporate securities, and interest rate swaps.

In having a so-called negative position in these cash equivalents and money-market securities, it is an indication of derivative use and short-term securities being put up as collateral as a way to boost leverage and increase the fund's holdings in bonds with longer maturities such as mortgage-backed securities, Treasuries and corporate bonds.